Method and device for establishing communication links and detecting interference between mobile nodes in a communication system

ABSTRACT

A wireless communication network includes a plurality of mobile nodes each including a transceiver, a phased array antenna connected to the transceiver, and a controller connected to the transceiver. The controller schedules a respective semi-permanent time slot for each time frame to establish a communication link with each neighboring mobile node and leaves at least one available time slot in each time frame. The controller also schedules the at least one available time slot to also serve the communication link with a neighboring mobile node based upon link communications demand. The phased array antenna is aimed by the controller towards each neighboring mobile node during communication therewith. The controller also detects interference in time slots for communication with neighboring mobile nodes and coordinates the scheduling of time slots based upon detected interference.

RELATED APPLICATION

This application is a continuation-in-part of copending application Ser.No. 10/043,457 filed Jan. 10, 2002, the entire disclosure of which isincorporated herein by reference.

This invention was made with Government support under Contract NumberN00014-96-C-2063 awarded by the Naval Research Laboratory. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to the field of communications, and moreparticularly, to a network of mobile communication systems operatingwith directional antennas.

BACKGROUND OF THE INVENTION

Time division multiple access (TDMA) is one example of an access schemeused for establishing communication links between wireless mobilecommunication systems. Communication links between the wireless mobilecommunication systems are established within a series of time frames.Each time frame is divided into time slots, with each wireless mobilecommunication system being assigned at least one time slot.

An omni-directional antenna is typically used by a wireless mobilecommunication system so that information transmitted by one mobilecommunication system is received by all the other mobile communicationsystems. When the mobile communication systems are operating at a fixedfrequency, they must take turns transmitting within their respectivetime slots to prevent channel interference.

To improve quality of a communications link between two wirelesscommunication systems, a directional antenna may be used. Thedirectional antenna provides an increased antenna gain in a desired areathat is limited in coverage while decreasing the antenna gain towardsthe remaining area.

U.S. Pat. No. 5,767,807 to Pritchett discloses phased array antennasbeing used for establishing communication links within a network ofwireless communication systems. The phased array antenna includesparasitic elements for selectively controlling the antenna pattern. Thephased array antenna radiates an omni-directional signal when all of theparasitic elements are in a high impedance state, and radiates adirectional signal when a selected number of parasitic elements areplaced in a lower impedance state in response to switching circuits.

More particularly, the Pritchett '807 patent discloses the acquisition,by a fixed initiating wireless communication system from a fixedreceiving wireless communication system, of a list of the wirelesscommunication systems operating in the network and a correspondingrespective time slot list for each wireless communication system. Atable is then created based upon the list for scheduling time slotsamong the wireless communication systems.

Scheduling time slots for wireless communication systems operating withdirectional antennas, particularly when the wireless communicationsystems are mobile, is complex. In such a dynamic network, mobilecommunication systems are continuously entering into and dropping out ofthe network. Furthermore, procedures for interference detection andavoidance are needed.

SUMMARY OF THE INVENTION

It is an object of the present invention to schedule time slots anddetect interference in a manner that is responsive to variations incommunication link demands in a wireless mobile ad hoc network.

This and other objects, advantages and features in accordance with thepresent invention are provided by a wireless communication networkincluding a plurality of mobile nodes each having a transceiver, adirectional antenna, e.g. a phased array antenna, connected to thetransceiver, and a controller connected to the transceiver. Thecontroller includes a time slot scheduling unit to schedule time slotsto establish a communication link with each neighboring mobile node, anantenna aiming unit to aim the directional antenna toward eachneighboring mobile node during communication therewith, and aninterference detection unit to detect interference in time slots forcommunication with neighboring mobile nodes. The controller coordinatesthe scheduling of time slots based upon detected interference. The timeslot scheduling unit may include a first time slot unit to schedule arespective semi-permanent time slot for each time frame to establish acommunication link with each neighboring mobile node and leaving atleast one available time slot in each time frame, and a second time slotunit to schedule the at least one available time slot to also serve thecommunication link with a neighboring mobile node based upon linkcommunications demand.

The interference detection unit may measure a signal-to-interferenceratio and/or a packet error rate. The packet error rate may be basedupon cyclic redundancy check (CRC) failures. Also, the interferencedetection unit may compare detected interference to a threshold.Preferably, the controller switches an order of communication betweennodes for a given time slot based upon detected interference, and mayalso coordinate scheduling of new time slots based upon detectedinterference after switching the order of communication. A plurality ofcommunication links may be established within a scheduled semi-permanenttime slot, with each communication link including a different pair ofneighboring mobile nodes.

Objects, advantages and features in accordance with the presentinvention are also provided by a method for establishing communicationlinks for the plurality of mobile nodes. The method includes, for eachmobile node, scheduling a respective semi-permanent time slot for eachtime frame to establish a communication link with a neighboring mobilenode and leaving at least one available time slot in each time frame,and scheduling the at least one available time slot to also serve thecommunication link with a neighboring mobile node based upon linkcommunications demand. The directional antenna is aimed toward eachneighboring mobile node during communication therewith, interference isdetected in time slots for communication with neighboring mobile nodes,and the scheduling of new time slots is coordinated based upon detectedinterference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a wireless mobile ad hoc network inaccordance with the present invention.

FIG. 2 is a more detailed block diagram illustrating a wireless mobilenode in accordance with the present invention.

FIG. 3 is a diagram illustrating a frame of time slots in accordancewith the present invention.

FIG. 4 illustrates the scheduling of available time slots to the networkdiagram illustrated in FIG. 2 in accordance with the present invention.

FIG. 5 is a top-level state diagram for the scheduling of semi-permanenttime slots and available time slots in accordance with the presentinvention.

FIG. 6 is a diagram illustrating a semi-permanent time slot schedulingprocess in accordance with the present invention.

FIG. 7 is a diagram illustrating a semi-permanent time slot beingscheduled for a new communication link in accordance with the presentinvention.

FIG. 8 is a diagram illustrating an available time slot schedulingprocess in accordance with the present invention.

FIG. 9 is a diagram illustrating an available time slot being added to acommunications link in accordance with the present invention.

FIGS. 10 and 11 are diagrams illustrating a semi-permanent time slotbeing scheduled for a new communications link based upon multiplesimultaneous antenna beams from a phased array antenna in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings in which preferred embodiments ofthe invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout and prime notations are usedin alternate embodiments. The dimensions of layers and regions may beexaggerated in the figures for greater clarity.

Referring initially to FIGS. 1-2, a wireless mobile communicationnetwork 10 comprises a plurality of wireless mobile nodes 12 a-12 h.Each mobile node 12 a-12 h comprises a transceiver 14, a directionalantenna 16 connected to the transceiver, and a controller 18 connectedto the transceiver.

The controller 18 includes a semi-permanent time slot unit 18 a forscheduling a respective semi-permanent time slot for each time frame forestablishing a communication link with each neighboring mobile nodewhile leaving at least one available time slot in each time frame. Anavailable time slot unit 18 b schedules the at least one available timeslot to also serve the communication link with a neighboring mobile nodebased upon link communications demand. In addition, the controller 18includes an antenna aiming unit 18 c for aiming the directional antennatoward each neighboring mobile node during communication therewith.

Parallel operations can reduce time slot allocation delay. Accordingly,the semi-permanent time slot unit 18 a may initiate one or moresemi-permanent time slot requests for respective time frames toestablish the communication link with each neighboring mobile node andleaving at least one available time slot in each time frame, whileprocessing multiple received semi-permanent time slot requests fromneighboring mobile nodes. The available time slot unit 18 b may initiateone or more available time slot request to also serve the communicationlink with the neighboring mobile node based upon link communicationsdemand, while processing multiple received available time slot requestsfrom neighboring mobile nodes.

In other words, a node can have one or more pending demand availablerequests and semi-permanent requests that it initiated while processingmultiple received requests. This may sometimes result in temporarilyallocating a given time slot to more than one neighbor. However, thisconflict may be eliminated by confirmation messages which indicate theselection of one neighbor node for the time slot, as is discussed inmore detail below.

Reliable confirmation messages may be provided with a couple ofdifferent approaches. An initiating mobile node transmits a request fortime slots to the receiving mobile node, which transmits a reply to theinitiating mobile node. The initiating mobile node transmits aconfirmation to the receiving mobile node, and the receiving mobile nodetransmits the reply again if the confirmation is not received.Alternatively, the receiving mobile node may transmit an acknowledgmentto the initiating mobile node, and the initiating mobile node transmitsthe confirmation again if the acknowledgment is not received.

If two nodes simultaneously initiate time slot requests to each other,the time slot request collisions should be handled reliably. Thecontroller 18 waits a time period to resend another request uponsimultaneously sending a request to another mobile node and receiving arequest from the other mobile node without receiving a correspondingreply. During this time period, incoming time slot requests may beprocessed. After the period ends, a new request may be sent to the othernode if no request was already received from that node or if no timeslot allocation was made to that node. If a delayed request reaches thefront of the queue, the controller 18 checks to see if a time slotallocation has already been made to that node. If so, the delayedrequest is discarded. Also, if the request is destined for a node thatis no longer a neighbor by the time the request reaches the front of thequeue, the delayed request is discarded.

An interference detection unit 18 d is included to detect interferencein time slots for communication with neighboring mobile nodes. Thecontroller 18 coordinates the scheduling of time slots based upondetected interference. The interference detection unit 18 d may measurea signal-to-interference ratio and/or a packet error rate. The packeterror rate may be based upon cyclic redundancy check (CRC) failures.Also, the interference detection unit 18 d may compare detectedinterference to a threshold. Preferably, the controller switches anorder of communication between nodes for a given time slot based upondetected interference, and may also coordinate scheduling of new timeslots based upon detected interference after switching the order ofcommunication.

A traffic coordination unit 18 e coordinates communication with eachneighboring mobile node by allocating time slots to the time slot unitbased upon link communications demand. The controller 18 coordinates thescheduling of time slots based upon based upon allocated time slots. Thetraffic coordination unit 18 e may allocate a bulk set of time slotsbased upon an increased link communications demand, and/or may request abulk set of time slots from neighboring mobile nodes based upon anincreased link communications demand. Also, the traffic coordinationunit 18 e may increase a maximum number of time slots, reallocate timeslots, and/or allocate half time slots based upon increased linkcommunications demand. Such increased link communications demand mayinclude streaming video and/or high rate sensor data.

The wireless mobile nodes 12 a-12 h are operating in a mobileenvironment. These systems may be ground based and/or airborne, wherebythey are continuously entering into and dropping out of the network 10.The directional antenna 16 may be a phased array, a dish or hornantennas, for example. Transmission via a directional antenna 16 enablesthe RF signal to be focused in a desired direction.

By selectively controlling the direction of the antenna pattern betweena pair of wireless mobile communication systems for establishing acommunications link therebetween, additional communication links may beestablished between other wireless communication systems within the samescheduled semi-permanent time slot. This is illustrated by communicationlink 27 operating in time slot 1 between mobile nodes 12 c and 12 e, andcommunication link 29 also operating in time slot 1 between mobile nodes12 a and 12 b, as best illustrated in FIG. 1. This feature of thepresent invention advantageously allows the resources of the wirelessmobile communication network 10 to be better utilized.

The controller 18 limits the number of communication links for eachwireless mobile node 12 a-12 h within each time frame based upon a totalnumber of time slots within the frame. The advantage of limiting thenumber of communication links to a fraction of the total number of timeslots within the time frame significantly simplifies the scheduling oftime slots with neighboring nodes.

The number of communication links for each wireless mobile node 12 a-12h within each time frame is less than or equal to N, and the totalnumber of time slots within each frame is greater than or equal to 2N−1.In addition to simplifying the scheduling of time slots, this type ofdistributed scheduling avoids conflicts.

Distributed scheduling allows any two pair of wireless mobile nodes,such as 12 a and 12 b, for example, to schedule a semi-permanent timeslot without having to communicate with any other wireless mobile node.In other words, there is no centralized master/slave type ofcoordination with all of the wireless mobile nodes 12 a-12 h forscheduling the semi-permanent time slots. Since the time slots among thewireless mobile nodes 12 a-12 h are scheduled in a distributed fashion,there is no single point of failure in the wireless mobile communicationnetwork 10.

The controller 18 may prioritize the communication links and drop one ofthe communication links based upon the prioritization for makingavailable a semi-permanent time slot for establishing a communicationlink with a new neighboring mobile node. Prioritization of thecommunication links will be addressed in greater detail below. Inaddition, the controller 18 may also prioritize the communication linksand schedule the at least one available time slot based upon thisprioritization.

The controller 18 may also schedule one of the semi-permanent time slotsas an available time slot if a number of the communication links is lessthan N. This advantageously supports communication link demands on an asneeded basis for the existing communication links. However, thecontroller 18 may reschedule the demand assigned time slot back to asemi-permanent time slot if the number of the communication links isagain equal to N, as will also be discussed in greater detail below.

Each communication link is formed by an initiating mobile node, such asnode 12 a, and a receiving mobile node, such as node 12 b, and theinitiating mobile node transmits a list of available semi-permanent timeslots to the receiving mobile node. The receiving mobile node 12 b thentransmits selection of one of the semi-permanent time slots to theinitiating mobile node. The initiating mobile node 12 a then confirmsselection of the selected semi-permanent time slot to the receivingmobile node.

Each mobile node may further comprise an omni-directional directionalantenna 20 connected to the transceiver 14 for exchanging positionalinformation with other neighboring mobile nodes. Other information thatmay be exchanged includes resource requirements and detection of thepresence of a potential new neighbor node. In addition, the phased arrayantenna 16 may simultaneously generate multiple antenna beams, whereinthe controller 18 aims the phased array antenna to multiple neighboringmobile nodes within a scheduled semi-permanent time slot.

The interference detection unit 18 d detects and avoids interference forcollinear node pairs within the beamwidth and allocated the same timeslot. For example, referring to FIG. 1, nodes 12 a and 12 e transmittingto nodes 12 b and 12 c, respectively during their half of the sameassigned time slot 1. With wide enough antenna beamwidths, both nodes 12b and 12c may simultaneously hear transmissions from both nodes 12 a and12 e. The interference detection unit 18 d may measure theSignal-to-Interference Ratio (SINR) at the physical layer during timeslot usage. Alternatively, the packet error rate can be measured at thelink layer based upon CRC check failures. If these measurements violatea specified threshold, the slot may be declared bad.

However, because fading may cause a single slot to fail this test, itmay be desired to declare excessive interference in a slot if m of ntrials of this slot suffer degradation. At this point, the controller 18attempts to avoid the interference. The order of Tx/Rx at both ends ofthe link may be switched for the time slot. If such switching fails, anew time slot may be coordinated. Of course both of these changes shouldbe made probabilistically to reduce the likelihood that both node pairstry to make the same change at the same time and thus remain inconflict.

The traffic coordination unit 18 e manages unbalanced traffic loads thatmay be generated by streaming video or high rate sensor data.Coordination mechanisms are provided to permit each half-duplex link toallocate a time slot in any Tx/Rx split of traffic. Also, the maximumnumber of time slots may be increased to a number above the minimum tocreate more demand time slots. Subslotting would permit an effectiveincrease or decrease in the maximum number of time slots as nodes may“steal” subslots from a semi-permanent allocated time slot to reallocateto a demand time slot. Moreover, a reservation protocol could be usedtogether with link scheduling procedures to indicate allocation ofresources for a high rate stream at each node along a path from a sourceto a destination node by requesting and allocating a bulk set of timeslots and/or subslots at each node along the path to accommodate thehigh rate stream. For reserved resources, separate queues and a queueservice discipline may be necessary to insure that the capacity requiredby the stream is delivered.

The invention is also directed to a method for establishingcommunication links for a plurality of mobile nodes 12 a-12 h, with eachmobile node comprising a transceiver 14, a phased array antenna 16connected to the transceiver, and a controller 18 connected to thetransceiver. The method comprises for each mobile node 12 a-12 hscheduling a respective semi-permanent time slot for each time frame toestablish a communication link with a neighboring mobile node andleaving at least one available time slot in each time frame.

The at least one available time slot is preferably scheduled to servethe communication link with a neighboring mobile node based upon linkcommunications demand. The phased array antenna 16 is aimed toward eachneighboring mobile node 12 a-12 h during communication therewith. Eachtime frame may have up to N semi-permanent time slots and at least 2N−1available time slots.

The method may also include initiating one or more semi-permanent timeslot requests for respective time frames to establish a communicationlink with each neighboring mobile node and leaving at least oneavailable time slot in each time frame, while processing multiplereceived semi-permanent time slot requests from neighboring mobilenodes, and initiating at least one available time slot request to alsoserve the communication link with a neighboring mobile node based uponlink communications demand, while processing multiple received availabletime slot requests from neighboring mobile nodes.

The directional/phased array antenna 16 is aimed toward each neighboringmobile node 12 a-12 h during communication therewith, interference isdetected in time slots for communication with neighboring mobile nodes,and the scheduling of new time slots is coordinated based upon detectedinterference. The interference detection unit 18 d may measure asignal-to-interference ratio and/or a packet error rate. The packeterror rate may be based upon cyclic redundancy check (CRC) failures.Also, the interference detection unit 18 d may compare detectedinterference to a threshold. Preferably, the controller 18 switches anorder of communication between nodes for a given time slot based upondetected interference, and may also coordinate scheduling of new timeslots based upon detected interference after switching the order ofcommunication. Also, communication with each neighboring mobile node 12a-12 h may be coordinated by allocating time slots for scheduling basedupon link communications demand.

The method further includes having each node prioritize thecommunication links and drop one of the communication links based uponthe prioritization for making available a semi-permanent time slot forestablishing a communication link with a new neighboring mobile node. Inaddition, an available time slot that is currently scheduled to serve aparticular communication link may be reassigned to another communicationlink based on link demand. This advantageously allows any mobile node toaccommodate variations in communication link demands.

Scheduling of the semi-permanent time slots and the available time slotswill now be discussed in greater detail. Details on steering thedirectional antennas 16 toward a receiving mobile node 12 a-12 h will beomitted since this feature of the present invention is readilyunderstood by one skilled in the art.

For purposes of discussion, it will be assumed that the directionalantenna 16 is a phased array antenna. As readily understood by oneskilled in the art, a phased array antenna 16 includes a plurality ofantenna elements and respective phase shifters that can be adjusted forproducing a steerable antenna beam in a desired direction. The phasedarray antenna 16 steers or scans the antenna pattern without physicallymoving the antenna.

Also for purposes of discussion, a number of assumptions about thewireless mobile communication network 10 are made. First, there is asingle frequency band that is a high data rate channel that is shared byall the wireless mobile nodes 12 a-12 h. This type of transmissionchannel is time shared between all the wireless mobile nodes 12 a-12 hfor both transmit and receive. All transmission slots are scheduled inadvance.

An assumption is also made that a separate low data rate overheadchannel is provided. This overhead channel can be used for nodediscovery, net entry, and exchange of various other data link controloverhead information including resource requests. This overhead channelis provided via an omni-directional antenna 20. Good global timingreference is also known at all nodes. The terms wireless mobile nodesand wireless mobile communications systems 12 a-12 h are interchangeablethroughout the following discussion.

The wireless mobile communication network 10 also includes thecapability for locating and tracking mobile nodes so that the phasedarray antennas 16 can be pointed accurately when a scheduled time slotis available. As noted above, a detailed discussion on thepointing/tracking will not be provided herein.

An assumption is also made that the phased array antennas 16 have zerobeamwidth. This assumption will be relaxed later. Consequently, we canassume that a transmission by a given mobile node will be received onlyby the neighbor mobile node to which it is attempting to transmit. Thisallows a less restrictive set of constraints on the scheduling of timeslots. Each communications link will be labeled with a number whichrepresents a scheduled time slot for transmitting and receiving datatherein.

The constraints are as follows. No node may have more than onecommunications link labeled with the same time slot number. A given timeslot assignment will apply to a half duplex link between two mobilenodes, and be used alternately by the two nodes for transmit andreceive. These two constraints imply that a time slot assigned by amobile node to one of its neighboring nodes is constrained by theprevious time slot assigned by that node to other links.

The scheduling of time slots for the phased array antenna 16 isillustrated in FIG. 1, which shows a network 10 with link connectivitybased upon scheduled time slots. The time slots are scheduled so thatthe wireless mobile nodes 12 a-12 h know when to point their respectivephased array antenna 16 toward a neighboring wireless mobile node.

The communication links are assumed to be bidirectional and are used ina half duplex fashion where each time slot number represents a time slotand a transmission opportunity in each direction occurring in that timeslot. The term N_(frame) will be used to denote the maximum link indexor the maximum number of time slots within a frame. In the case of thisexample, N_(frame)=6.

FIG. 3 illustrates a representative frame of time slots. In the simplestformulation, each epoch or frame has n slots and the value of n is setto N_(frame). In the figure we also show how a time slot is used for thelink connecting to nodes labeled as nodes A and B. Each time slot isdivided into two mini-slots 22 a, 22 b. The first mini-slot 22 a (e.g.,half of the time slot) is used for transmissions from node A to B. Thenthe direction of the link is reversed and the second mini-slot 22 b isused for transmissions from node B to A.

During the transmission periods, multiple packets can be transmitted. Asindicated, each mini-slot 22 a, 22 b also contains a guard time 24 a, 24b selected according to the following considerations. The maximum rangebetween any pair of nodes determines the maximum propagation delay thatmust be accommodated. A maximum range of 100 miles corresponds to about0.5 ms of propagation delay. A guard time is allocated for eachmini-slot 22 a, 22 b to accommodate uncertainty of propagation delay andunequal propagation delays between all pairs of nodes.

At a maximum range of 100 miles, a guard time of 0.5 ms is needed. Theguard time allocation for a maximum range of 100 miles implies the needto make the mini-slots 22 a, 22 b on the order of 2 to 4 ms to minimizethe channel efficiency loss. As an example, if we assume a 50 Mb/s datarate on the communication links and a maximum range of 100 miles, then a4 ms mini-slot implies 200,000 bits/mini-slot (250 mini-slots persecond). Then the mini-slot would contain a 25,000 bit guard time and175,000 bits of mission data.

The controller 18 may also bias each established link to assign prioritywhen the available time slots are scheduled. As will be discussed ingreater detail below, semi-permanent (SP) time slots and available ordemand assigned (DA) time slots are provided within each frame. A statedobjective is to increase reuse of time slots among several nodes at thesame time. While the mobile network 10 in FIG. 1 is limited in the totalnumber of nodes and communication links, there are a number of cases ofparallel usage of time slots. For example, time slots 1 and 2 aresimultaneously each used on 3 different communication links, and timeslot 6 is used on only one link. All the other time slots are assignedto two communication links. We can define a reuse factor which indicatesthe average level of reuse as a ratio of the total number of time slotassignments in the network (N_(frame)) to the number of assigned timeslots (Num_Slots_Assigned): $\begin{matrix}{R = \frac{{Num\_ Slots}{\_ Assigned}}{N_{frame}}} & (1)\end{matrix}$For the example network 10 in FIG. 1, the reuse approach provides areuse factor of R=14/6=2.333, indicating that on the average there areslightly more than two simultaneous users of each time slot in thenetwork schedule. It is obvious that the reuse factor calculated for anyspecific scheduling algorithm will be highly dependent on the networksize and topology. A full comparative evaluation should consider avariety of network sizes and topologies.

A lower bound on the value of N_(frame) for any graph can be determinedby noting that each node requires at least as many time slots as thenode has neighbors, i.e., the node requires a number of time slots atleast equal to its degree. Then N_(frame) must be at least as great asthe maximum node degree over the entire graph. Thus, denoting the degreeof node i by d_(i) the lower bound on N_(frame) isN _(frame)≧max_(i) {d _(i)}  (2)For the example network 10 illustrated in FIG. 2 the reuse portion isassigned the scheduling with N_(frame) equal to the minimum number oftime slots that must be used according to equation (2). Note thatseveral nodes, namely all nodes but node 1, are assigned less than thefull set of time slot. Thus, an enhanced scheduling algorithm may beable to assign additional slots to some of the links without introducingconflicts in scheduling.

The following discussion focuses primarily on the scheduling of timeslots for generating the link schedules. Other parts of the overallphased array network problem that ultimately must be addressedinclude: 1) node and neighbor discovery, 2) net entry, 3) overheadchannel format and protocol including protocol exchanges for schedulingupdates, and 4) tracking and location of neighbor nodes (may includeassistance of phased array antenna 16), and 5) a routing algorithm for adynamic network topology.

The approach for scheduling time slots according to the presentinvention is based upon the following principles. First, a specifiednumber of time slots are allocated as semi-permanent (SP) time slotsscheduled for a given link. The rest of the available time slots (DA)may be allocated on a demand-assigned basis to those nodes/links thatneed them most. This allows flexibility in shifting the schedule on anas needed basis. Secondly, as discussed above, a limit on the maximumnumber of semi-permanently assigned time slots is established. Thislimit is a parameter that is selected based upon a specific network.This limit is also the upper limit on the number of allowable neighbornodes, with a single SP time slot per node.

Third, as also discussed above, a limit on the maximum number of timeslots per frame is established. This limit is a parameter that is alsoselected based upon a specific network. This limit is important forestablishing a limit on latency since it determines the maximum revisittime for a link transmit opportunity.

Fourth, the relationship between the number of total time slots perframe, N_(frame), and the limit on the maximum number ofsemi-permanently assigned time slots per frame is chosen so that thescheduling of the semi-permanently assigned time slots is greatlysimplified and scheduling conflicts may be significantly avoided evenwith distributed scheduling.

By limiting the maximum number of semi-permanently assigned time slotsper node to a certain fraction to the total number of time slots perframe, the process of distributively assigning semi-permanently assignedtime slots is greatly simplified. The upper limit on the number of thesemi-permanently assigned time slots (and, therefore, the maximum numberof allowable neighbor nodes) will be denoted by N. We will considervalues of N_(frame) such that:N _(frame)≧2N−1  (3)

Assume that all nodes 12 a-12 h in the network 10 are connected bydirectional links, where each node has a single beam phased arrayantenna 16 with beam sharing by time hopping and pointing to itsneighbor nodes. Further, assume that the number of neighbors is equal toN, and the limit on the allowable number of semi-permanent time slots(with one SP time slot allocated per neighbor) is fixed.

If the fixed value of N_(frame) satisfies equation (3), then all nodescan select a different semi-permanent time slot for each of these linksby mutual agreement with the neighbor for that link without regard towhat links other nodes are selecting more than one-hop away. This allowseach node to select its semi-permanent time slot for the link to aneighbor node in a very direct fashion by communicating only with thatneighbor node. This process can be followed for up to N neighbor nodes.

The key is recognizing that as the value of N_(frame) increases for afixed value of N, there are fewer constraints on the ability of a nodeto select a time slot that does not conflict with a neighbor's choice ofa time slot. A node selecting a time slot for a new link must select atime slot that it is not currently being used and that the neighbor isnot currently using.

If a node currently has m neighbors with a single time slot assigned toeach of these links to the neighbors and is adding a link to a newneighbor node, then the neighbor node can be using at most (N−1) timeslots. Thus, if N_(frame) is greater than (m+N−1), then there will be atleast one more time slot available that the node can assign to the newlink. The worst case in this assignment process is when the node alreadyhas (N−1) neighbors and is assigning the time slot for the N^(th)neighbor node. In this case N_(frame) must satisfy equation (3) for anadditional time slot to be guaranteed to be available for assignment tothe link to the N^(th) neighbor.

Some additional observations will be made about how this property can beexploited in the disclosed time slot scheduling approach. First, a nodeneed only coordinate the selection of the semi-permanent time slot to beassigned for a directional link to a neighbor with that neighbor. Thenode requesting the link might, for example, send to the neighbor thelist of suggested time slots for the link. This is based upon those timeslots not being used for SP assignments. There could be some ordering ofthis list based upon other factors to be discussed below, but this isnot necessary. The neighbor node can then select from this list the timeslot it prefers and return a reply with this selection. This allows usto define a straightforward, fully distributed algorithm for schedulingthe semi-permanent time slots.

If a node has less than N neighbors, then more than one of its N allowedsemi-permanent time slots could be assigned on individual links.However, in this case there is no guarantee that all N assignments canbe made via neighbor-to-neighbor node coordination without someconflicts. For example, if N=6 and a node had only 3 neighbors but eachof these neighbors each had 6 neighbors, then the node would be able toassign only one time slot to each of the links with its 3 neighbors. Inorder to simplify our algorithm, we will not allow scheduling of morethan one SP time slot per link. However, all unused time slots may beallocated as available time slots.

For certain networks with very large numbers of nodes where the numberof potential neighbors will be much larger than the limit N, there willalso be a topology control problem to deal with. The node will be facedwith choosing, from among the potential neighbors, those neighbors thatcreate the optimum network topology. This topology control problem alsois related to the concept of optimizing an energy efficient network. Inthe case where the number of potential neighbors is much larger than thelimit N, a topology control function can be used to select the neighbornode to connect to.

If we assign to N_(frame) the minimum value allowed by (3), then eachnode will be allowed to have a maximum of N semi-permanent time slotsand a total of (2N−1) time slot assignments. The demand assigned timeslots will be assigned on a basis to best accommodate the traffic load.Of course, assigning a much larger value of N_(frame) is also an option.In this case, there will be many more time slots available for demandassignment. There may be applications for which this is a desirable wayto configure the network.

As with the semi-permanent time slots, the node need only coordinate theselection of the available time slots to be assigned for a directionallink to a neighbor with that neighbor. This means that a neighbor willsend a request to the neighbor for the time slot assignment over thedirectional link, and receive either a grant of the assignment or adenial of the request over the same link.

A node requesting the allocation of an available time slot DA from aneighbor node will do so based upon a perceived need for additionalcapacity on that link. This may be prompted by a high link utilization(queue buildup) based on short and long term measurements. The requestwill contain the number of slots requested and a metric, which indicatesthe priority to be attached to the request. The metric might indicatethe queue length as a measure of the need for the time slot allocation.

The node receiving the request may also receive requests from otherneighbor nodes, which may contend for allocation of the same time slot.In order to simplify the protocol, a node must complete processing onethread of an available time slot DA allocation before considering thenext allocation. These allocations may not persist for a long period oftime because they are constantly subject to preemption to becomereallocated as semi-permanent time slots as a result of topology changesor subject to reallocation due to shifting traffic demand.

Neighbor and link discovery will now be discussed. The distributed linkscheduling algorithm requires support from an omni-directional overheadchannel for certain protocol exchanges that must occur with a potentialneighbor node prior to the establishment of the directional link withthat node. Such messages include the REQ_SPTS which requests theallocation of a semi-permanent time slot on the directional link to thatnode.

In addition to supporting protocol message exchanges which directlysupport the protocol defined herein, the omni-directional overheadchannel must support the function of neighbor and link discovery. Thisis usually done through periodic omni transmissions by each node via anomni-directional antenna 20 that alerts any other node that move withinrange that the two nodes can be neighbor nodes. Several ad hoc routingprotocols (including OLSR) have defined such a supporting protocol.These previously defined protocols could be adapted to support thisdistributed link scheduling algorithm. The primary function that must beperformed by such a protocol is to discover new potential neighbor nodesand to report these to the topology control function.

One approach for node and link discovery includes each node periodicallytransmitting beacon messages over the control channel to notify neighbornodes of its presence and its position. In addition, link state messagesare transmitted periodically to notify neighbor nodes of the identity ofits beacon neighbors (BN list) and its PA neighbor nodes (PAN list) andthe time slots assigned to these nodes.

The link discovery portion of the algorithm continually compares thebi-directional beacon neighbors (BBN) list with the PAN list to see ifthere are any nodes on the BBN list that are not on the PAN list. Anysuch neighbor node becomes a candidate for link testing to determine ifa PA link is possible. According to this approach, after an exchange ofcontrol messages the directional link is tested to determine if reliablecommunication is possible. If communication is reliable, the newneighbor node is added to the PAN list.

This validates communication in the testing time slot, but notnecessarily in the time slot that may be assigned to the link on asemi-permanent basis. One approach is to do it this way or anotherapproach is to wait until an SP time slot is assigned and test it inthis time slot.

The topology control function can be a very straightforward function ifit does not have to do topology optimization. The purpose of thisfunction is to take the list of nodes in the PAN list, the informationabout the reliability of these links, and the information about thenetwork topology, and use this information to determine which nodes onthe PAN list should become PA neighbors. This is the function thatshould optimize the network topology if there are constraints such asthe number of PA neighbors that do not allow all nodes in the PAN listto become PA neighbors.

With the proposed constraints of a fixed value for N_(frame) and a fixedvalue for N (the maximum number of semi-permanent time slots per node),the potential exists for having some concern about network topologyutilization. This would certainly be the case if these values wereselected to be very small numbers. For example, if N=3 were selectedwith N_(frame)=5, it may be difficult to expect a well connected networktopology when we could have no more than 3 neighbors for any node,unless an intelligent topology control function carefully utilized thetopology prior to adding new PA neighbor nodes. This may be particularlyso for a large network.

Thus, the topology control function should create a neighbor priority(NP) list, which is the PAN list ordered in order of desirability aspotential PA neighbors. This list will direct the priority order inwhich potential PA neighbors are scheduled time slots. However, ourinitial problem is that of a small network with perhaps 15 nodes. Inthis case, we could specify N to have a value in the range of 5 to 8 andstill have low latency. There is very little likelihood that there willbe any topology utilization issues since allowing for 5 to 8 neighbornodes will allow almost all possible neighbors to be PA neighbors.

A second purpose of the topology control function is to generate thetopology change event that causes the link scheduler process to changestate and perform the reallocation process for the SP time slots.

A top-level scheduling algorithm structure will now be discussed. Thescheduling process was formulated with the objective of minimizing thecomplexity of the process while taking advantage of the overall approachoutlined above. A key to controlling this scheduling is maintaining anaccurate data structure at each node reflecting the state of time slotschedules for future time slots assigned to the link with each neighbornode.

Two data structures are proposed: a slot assignment DB and a linkmessage DB. The possible states of links in the data structure for agiven time slot in the epoch are listed in TABLE 1. The table describeseach possible state and gives the notation for that state. TABLE 2 showsan example slot assignment DA and the contents indicating the timeslotsfor N_(frame)=9 (N=5), the state assignments for each state, and exampleassigned neighbor IDs for each time slot.

In this example, 4 neighbors have been assigned SP time slots so oneadditional neighbor may be connected with these constraints. There isone free time slot which may be allocated as a DB time slot or offeredwith the DB time slots to be allocated as an SP time slot if a newneighbor node is possible. The use of the link message DB will bediscussed later in the detailed protocol explanation. The example alsoindicates the use of sub-slots, e.g., 2 sub-slots per slot.

This is a concept to be used with the DA allocations to allow finergranularity. The meaning in this case would be that an allocation oftime slot k, sub-slot 1 would be an allocation to a link of time slot kon the odd numbered frames. Conversely, sub-slot 2 would indicate anallocation of the time slot on the even numbered frames.

TABLE 1 Time Slot State in DB Notation Free Free SP Allocated Time SlotSP_Alloc DA Allocated Time Slot (May DA_Alloc Be Preempted by SPAllocation Process or by DA Reallocation) SP Allocation Request MessageSP_Req Sent SP Allocation Reply Message SP_Reply Sent DA AllocationRequest Message DA_Req Sent (May Be Preempted by SP Allocation Processor by DA Reallocation) DA Allocation Reply Message DA_Reply Sent (May BePreempted by SP Allocation Process or by DA Reallocation)

TABLE 2 Assigned Time Slot Subslot State Neighbor ID 1 — Free — 2 —SP_Alloc 3 3 SP_Req 4 4 1 DA_Alloc 3 4 2 DA_Alloc 4 5 1 DA_Alloc 5 5 2DA_Alloc 3 6 — SP_Alloc 5 7 1, 2 DA_Alloc 8 8 2 DA_Alloc 4 9 — SP_Alloc8

The top-level state diagram for the link scheduling protocol is shown inFIG. 5. The diagram shows two independent processes 30 and 32 that areresponsible for maintaining and modifying the time slot allocationdatabase. On the left side is the state diagram for the process formaintaining and assigning semi-permanent (SP) time slots, i.e., process30. This process has priority over the assignments made by the process32 on the right, which has responsibility for assigning the available(DA) time slots. Within process path 31, the time slots that can beseized are as follows: free, DA allocated, and in process of being DAallocated. Similarly, within process path 33, the time slots that can beseized are as follows: free, DA allocated and also need to bereallocated.

This database must be controlled as a locked database such that for anygiven time slot assignment state, only one of the two schedulingprocesses may modify that state at a given point in time. Once one ofthe processes begins to modify the state of a particular time slotassignment, the state is locked and the other process may not modify ituntil it is released.

At any time each time slot in the DB is in one of seven states asindicated in TABLE 1. Available time slots are said to be in the freestate, i.e., they are not assigned to a link to one of its neighbornodes either because a scheduling conflict has prevented assignment orbecause the time slot has recently become free and has not yet beenscheduled.

As indicated, a time slot in the free state may be scheduled either asan SP time slot or a DA time slot. A time slot that has been allocatedas SP assigned may be modified only by the process that maintains SPtime slots. The time slot may be deallocated by this process if networktopology changes or if a more desirable topology is possible. Until sucha time slot is returned to the free state, the process for maintainingand assigning the DA time slots cannot modify its state.

In addition, any time slot with a DB state indicating that it is in theprocess of being SP assigned cannot be allocated by the DA assignmentprocess. This includes states indicating that SP request and replymessages have been sent. However, if the state of a time slot is DAallocated, then it may be reallocated by the DA assignment process. Thismight be done if the loading on the network indicated that areallocation of the DA time slot is needed.

In contrast, the process allocating SP time slots has priority. Inaddition to assigning free slots, it may seize and reassign all timeslots that have been DA assigned or are in the process of being DAassigned. This is done to provide a straightforward process of ensuringat least a single SP time slot assigned to each neighbor node during aframe of N_(frame) time slots. SP allocated time slots are returned tothe free state only if the link is lost or if the topology controlfunction determines that a particular link should no longer be in thelist of the top N links to be established with neighbor nodes.

FIG. 5 illustrates how this process works at the top level. The SP slotassignment process has greater flexibility in allocating time slots. Itcan seize more time slots for allocation than the DA process, and it canseize time slots that either have been DA allocated or are in theprocess of being DA allocated. The SP process may receive various eventsfor processing including topology change events from the topologycontrol function and protocol messages.

Such events might include loss of link to a neighbor, discovery of a newneighbor, reception of an SP allocation request message from a neighbornode, and the discovery that a topology change should occur to eitheradd a link to a neighbor, break a link, or do both. The topology changeevent notification will carry data that will describe the topologychange that needs to occur.

If the event described a loss of a link, then the only action that mustbe taken is to change the appropriate time slot state in the slotassignment DB to “free.” If a link is to be added the process is morecomplex. In this case, the SP slot assignment process initiates protocolmessage exchanges with the new neighbor node and modifies the slotassignment DB. This ultimately results in the agreement between the twonodes on a time slot assignment for the SP slot assigned to this link.Only a single SP time slot is to be assigned to each link with aneighbor to simplify the protocol. Additional details of this protocolare described below.

The process of assigning DA time slots follows a similar procedure. TheDA slot assignment process must calculate the DA time slot needs andcompare them with the allocated time slots to determine if a new timeslot reallocation is needed. If a reassignment of DA slots is initiated,it will also lead to a series of protocol message exchanges withneighbor nodes to agree on the reassigned time slots. The DA slotassignment process may reassign only time slots that are in the freestate or not SP assigned. More about the protocol details and theprocess for determining when DA time slot reassignment is needed will bediscussed below.

Allocating semi-permanent time slots to directional links will now bediscussed. In the description of the approach for allocating Nsemi-permanent time slots assume that N is fixed and intelligentlychosen with respect to the network size and environment. Also assumethat N_(frame)=2N−1. N_(frame) could also be set at any value higherthan this to provide additional on-demand time slots if that is deemedto be useful for the particular network and traffic environment.

Several important functions are provided by the topology controlfunction. The neighbor priority (NP) list is generated by the topologycontrol function and is used to indicate the preferred PA neighbor nodesfor the assignment of time slots.

If the length of the NP list is N or smaller, then the topology controlfunction will generate topology change events to the SP slot assignmentprocess to make it attempt to get time slot assignments to all of theseneighbor nodes. If the length of the NP list is greater than N, then itwill generate topology change events to the SP slot assignment processto obtain time slot assignments to each of the N highest priority nodeson the NP list.

The NP list is constantly changing due to network dynamics. When PAlinks go down, the node is removed from the NP list and the time slot(s)for that link are then subject to reallocation. This is initiated by thetopology control function which sends the SP slot assignment process alink delete event. Thus, the SP time slot and any DA time slotsallocated to that link become available for reallocation to another nodeon the PA list.

The first choice when slots become available is to allocate the slot(s)to additional PA neighbor nodes if that is possible given the currentstate of the NP list. If no additional neighbor nodes can be added, thenthe slot(s) can be reallocated on a DA basis.

FIG. 6 shows a state diagram of the SP slot assignment process. In orderto manage the protocol message processing, a link scheduling message DBis created as shown in TABLE 3. This maintains the state needed fromprior protocol exchanges to be used when the next SP message arrives forprocessing. The idle process does event management in that it checksreceived events prior to allowing a state change to one of the otherstates.

These operations include checking received messages to determine if theyare consistent with the current state of the DB. If a message isinconsistent with the DB, it is discarded. Certain timeouts may indicatethat DB state needs to be reset. This process performs this function.

TABLE 3 Time Selected Link Time Slot Time Selected Num_(—) Nbr_ID Stateout List Slot Subslot tries 1 SP_Alloc — — 2 1 — 1 SP_Alloc — — 2 2 — 1DA_Alloc — — 5 1 — 2 SP_Alloc — — 4 1 — 2 SP_Alloc — — 4 2 — 2 DA_Alloc— — 5 2 — 3 SP_Req T2 Ls — 1 4 SP_Alloc — — 6 1 — 4 SP_Alloc — — 6 2 —

There are four basic message types required in the SP time slotassignment protocol as listed below in Table 4. The use of these areself-explanatory and consistent with the prior discussion.

TABLE 4 Message Type Message Function REQ_SPTS Request New SP SlotAllocation REPLY_SPTS Reply to Received REQ_SPTS CONFIRM Response toReceived REPLY_SPTS DELETE_TS Message Indicating Deleted Time SlotAllocation

An example of SP time slot assignment is shown in FIG. 7. Nodes 1 and 2both have 3 neighbors with the SP time slots allocations shown for eachlink. Therefore, they can add an additional link between themselves. Thelink scheduling protocol will find an acceptable time slot for the SPallocation. The corresponding protocol message exchange is shown inTABLE 5.

Node 1 initiates the exchange by sending a REQ_SPTS(L=(4,5,6,7)) with alist of at least N candidate slots. This list may include all free andDA time slots. Node 1 is using slots 1, 2 and 3 for SP allocations toits neighbors so its list L contains the other time slots 4, 5, 6 and 7.When the request message is sent, the appropriate changes are made tothe time slot and link scheduling message data structures. Node 2 isusing time slots 4, 5 and 6 as SP allocations for its links to its 3neighbors so it selects time slot 7 as the only one that will work forthe new link. It sends this choice in the reply message.

When a reply message is sent, the appropriate changes are also made tothe time slot and link scheduling message data structures. Finally, whena confirm is sent or received, the state of the appropriate time slotsare changed to “SP allocated to link (1,2).”

Note also that if nodes 1 and 2 had already selected 4 neighbor nodes,it would still be possible for them to find common time slots with whichto establish a link between them if they used the same time slots withat least two of their neighbors.

TABLE 5 Node 1 Node 2 Receives Link Add Event From Its Topology ControlFor A Link From Node 1 to Node 2 Send Msg Lost REQ_SPTS(L = (4, 5, 6,7)) → Timeout and retry Resend → Rcvd REQ_SPTS(L = (4, 5, 6, 7))REQ_SPTS(L = (4, 5, 6, 7)) Rcvd REPLY_SPTS(Slot 7) ← SendREPLY_SPTS(Slot 7) Send CONFIRM(Slot 7) → Rcvd CONFIRM(Slot 7) Slot 7Allocated to Link Slot 7 Allocated to Link (1, 2) (1, 2)

Some initial pseudocode describing the processes required in FIG. 6 hasbeen developed. There are various events that may occur which must beprocessed by the SP slot assignment process 34. Event management is donein the idle process as shown in TABLE 6. Four categories of events areshown: received message, check timeouts, link addition notification fromtopology control, and link failure or link deletion.

Received messages are first checked versus the link scheduling messageDB to insure that the message is consistent with the current state ofthe DB. For example, if we sent a request to a neighbor, the nextmessage expected is a reply. To simplify this distributed protocol, onlyone thread of SP protocol message exchanges is allowed at a time. Thisis enforced in the procedure by checking the DB to see if other SPmessage exchanges are ongoing prior to initiating a link add transitionor prior to processing a REQ_SPTS message.

If a link addition cannot be initiated because another SP protocolthread is currently in process, the link addition will be postponed bybacking off and rescheduling for a later time when the other process isexpected to be completed. Allowing multiple attempts is done to handlepotential conflict between several nodes attempting to add linkssimultaneously. This is not meant to deal with the problem of anunreliable RF link. This latter issue should be addressed by using alink protocol on the overhead channel that uses ARQ and retransmissionto recover lost/errored messages.

Thus, the distributed scheduling protocol can assume that messages willnot get lost. This allows simplification of the protocol. When topologycontrol selects a neighbor node from the NP list to connect to as a newneighbor, it issues a topology change (link addition) event which (afterconsistency checks in the idle process) causes a transition to the linkadd state in the SP slot assignment process.

TABLE 6 Procedure for Idle State (SP Event Management) Case Event Type Received Message:   If received message is not consistent with the   state of the Link Scheduling Message    DB for that Nbr_ID    DiscardMessage   Elseif message type = REQ_SPTS    If no pending SP messageactivity in the Link     Scheduling Message DB for link additions    other than receiving a previous REQ_SPTS     message from Nbr_ID    Transition to Process REQ_SPTS state to     process message    Else    Reject new link and send negative     REPLY_SPTS message to Nbr_ID   End   Elseif message type = REPLY_SPTS    Transition to ProcessREPLY_SPTS state to    process message   Elseif message type = CONFIRM   Transition to Process CONFIRM state to    process message   Elseifmessage type = DELETE_TS    Transition to Process DELETE_TS state to   process message   End  Check Timeouts:   Check all timeouts   IfTimeout expired for a link in the SP_Req state    Transition to Link AddState   If Timeout expired for a link in the SP_Reply    state    ResetSlot Assignment DB for time slot Ns and    in the Link Message state in   Link Scheduling Message DB for index Nbr_ID   End  Link AdditionNotification from Topology Control:   If no pending SP message activityin the Link    Scheduling Message DB    Transition to Link Add state toadd Nbr_ID   Else    Backoff and reschedule Link Addition   End  LinkFailure or Link Deletion:   Transition to Link Delete state to deletelink to   Nbr_ID   End End

Psuedocode for the link add process is shown in TABLE 7. This starts aprocess which requires coordination of the SP time slot assignment andprotocol message exchanges between only the two neighbor nodes. The noderequesting the link sends a REQ_SPTS message to the candidate neighbornode with the list of acceptable time slots for the link.

The list of candidate time slots must contain at least N time slotsincluding at least one semi-permanent time slot SP. The list can alsoinclude possibly all of the N−1 available DA time slots. The availableor on-demand time slots may be currently temporarily allocated foron-demand traffic. This list will be priority-ordered to indicate thetime slot preference that causes the least perturbation in the currentavailable time slot assignments. In other words, the notation being usedis that a time slot is not an SP time slot unless already allocated to acommunication link. Any of the 2N−1 time slots may be an SP time slot.Thus, the list of N time slots sent are all either free time slots or anavailable DA time slot. These may be N−1 SP time slots-but they arealready allocated and are not on the list.

The REQ_SPTS message can be sent up to MAX_TRIES times to allow forunreliable links and conflicts with other assignments potentiallyoccurring simultaneously. The timeout in the link scheduling message DBtriggers the retries if there is no REPLY_SPTS message from the neighbornode in response to the REQ_SPTS message. Once the REQ_SPTS message issent the process returns to the idle state where other events can beprocessed.

TABLE 7 Procedure for Link Addition to Node Nbr_ID (Generate REQ_SPTSMessage) If Num_tries = MAX_TRIES (No more tries)  Reset state of LinkScheduling Message DB for index  Nbr_ID (Link State = Free and notimeout for retry)  Return to Idle state Else  If initial try to nodeNbr_ID   Set Num_tries = 1 in Link Scheduling Message DB   for indexNbr_ID  Else   Set Num_tries = Num_tries +1 in Link Scheduling   MessageDB for index Nbr_ID  End  Construct list Ls of time slots to offer toNbr_ID  Append list Ls to REQ_SPTS message and send to Nbr_ID  Setuptimeout and Link Message state in Link Scheduling  Message DB for index   Nbr_ID and in Slot Assignment DB  Return to Idle state End

The neighbor receiving a REQ_SPTS message will have its SP slotassignment process transition to the process REQ_SPTS state. Theprocedure for processing this message is shown in TABLE 8. Thisprocedure takes the offered list of time slots, Ls, and selects itspreferred time slot, Ns.

If the number of links to neighbor nodes, Num_links, is less than thelimit N, the procedure selects the time slot it prefers from this list.Then a REPLY_SPTS reply message with this selection is sent. If the linkcannot be accepted or if there is another ongoing SP slot assignment inprocess, a negative REPLY_SPTS reply message is sent.

The selected time slot will be selected from one of its N available timeslots or one of its free time slots. An available time slot is either a“free” time slot or an available DA time slot. There will be at least Nof these if we can add another link. Each node always manages its timeslots so that there are N time slots available to assign assemi-permanent time slots (one to each of N neighbor nodes if that manyneighbor nodes are available). If it accepts the link, then it will haveat most N−1 other neighbor nodes with one semi-permanent time slotallocated per node. The procedure also makes the appropriatemodifications to the state in the link scheduling message DB and theslot assignment DB.

TABLE 8 Procedure for Processing REQ_SPTS Message (from Nbr_ID) IfNum_links<N  Examine list Ls of the available time slots received  frompotential neighbor node   Nbr_ID, compare with the current allocationsin   the Slot Assignment DB, and select the best   assignment = Ns  Makeappropriate modification to the Slot Assignment DB  (mark it asSP_Reply) for time slot Ns  If time slot Ns was DA allocated   SendDELETE_TS to the neighbor node allocated the   DA time slot  End  Appendtime slot choice, Ns, to REPLY_SPTS message and  send to Nbr_ID  Setuptimeout and Link Message state (to SP_Reply with  time slot Ns) in Link  Scheduling Message DB for index Nbr_ID  Return to Idle state Else Reject new link and send negative REPLY_SPTS message to  Nbr_ID  Returnto Idle state End

A received REPLY_SPTS message is processed as shown in TABLE 9. Thechoice of time slot, Ns, received from the neighbor node is extractedfrom the message. We will also require the node to confirm this replywith either a positive or negative CONFIRM message that indicates thatit will agree to use the allocated time slot. This three-way handshakeeliminates uncertainty in the outcome of the scheduling process.

If the REPLY_SPTS message is a positive reply, then the choice of timeslot, Ns, is examined to see if it is still an allowable assignment fora new SP time slot for the new link. If it is allowable, then theappropriate modifications to the state in the slot assignment and linkscheduling message databases are made. Then a positive CONFIRM messageis returned.

If the received REPLY_SPTS message was negative, then the slotassignment and link scheduling message databases are reset for thisNbr_ID. Otherwise, if the choice of Ns is no longer allowable, then thelink scheduling message database is reset for this Nbr_ID. Then anegative CONFIRM message is sent to the neighbor node rejecting thelink.

TABLE 9 Procedure for Processing REPLY_SPTS Message from Nbr_ID Extracttime slot choice Ns from the REPLY_SPTS message from Nbr_ID If (positiveREPLY_SPTS message) and (choice of Ns is still  allowable from SlotAssignment DB)  Make appropriate modification to the Slot Assignment DB (mark it as SP_Reply)   for time slot Ns and in the Link Message statein   Link Scheduling Message DB   for index Nbr_ID  If time slot Ns wasDA allocated   Send DELETE_TS to the neighbor node allocated the   DAtime slot  End  Create CONFIRM message for Ns and send to Nbr_ID Increment Num_links  Return to Idle state Elseif negative REPLY_SPTSmessage  Reset Slot Assignment DB for time slot Ns and in the  LinkMessage state in   Link Scheduling Message DB for index Nbr_ID  Returnto Idle state Else  Reset Link Message state in Link Scheduling MessageDB  for index Nbr_ID  Send negative CONFIRM message to Nbr_ID  Return toIdle state End

Table 10 shows the procedure for processing CONFIRM messages. If theCONFIRM is positive, the link is considered to be added to the set ofneighbors. The number of links for the node, Num_links, is incremented.The assigned time slot, Ns, is marked SP_Alloc in the slot assignmentDB, and the link message state in the link scheduling message DB isreset for index Nbr_ID. If the message was a negative CONFIRM, then theslot assignment and link scheduling message databases are reset for thisNbr_ID.

TABLE 10 Procedure for Processing CONFIRM Message from Nbr_ID Ifpositive CONFIRM message  Make appropriate modification to the SlotAssignment DB  (mark it as SP_Alloc)   for time slot Ns  Reset LinkMessage state in Link Scheduling Message DB  for index Nbr_ID  IncrementNum_links  Return to Idle state Else  Reset the Slot Assignment DB (markit as Free) for time  slot Ns  Reset Link Message state in LinkScheduling Message DB  for index Nbr_ID  Return to Idle state End

An allocated time slot may need to be deallocated for one of severalreasons. If during the course of normal operation a link goes down orbecomes unreliable, then the topology control function gets involved toaddress the unreliable link problem. ultimately, it may generate atopology change (e.g., link deletion) event directing the SP slotassignment process to delete all slots assigned to the link.

The steps involved in this procedure are shown in TABLE 11. The link isde-allocated by sending a DELETE_TS message from the node requesting thede-allocation of all the time slots which are shared with the othernode. In addition, the appropriate entries in the link schedulingmessage DB and the slot assignment DB are reset.

TABLE 11 Procedure for Link Deletion to Node Nbr_ID (Generate DELETE_TSMessage) Extract list of all SP and DA time slots, Ls, from the SlotAssignment DB assigned to the  link to Nbr_ID Construct message,DELETE_TS, with the list, Ls, and send to Nbr_ID Reset Link SchedulingMessage DB for index Nbr_ID and Slot Assignment DB for all time slots inLs Decrement Num_links Return to Idle state

Table 12 shows the procedure for processing a received DELETE_TSmessage. The list of deallocated time slots, Ls, is extracted from themessage. Then the appropriate state in the slot assignment DB and in thelink scheduling message DB is reset.

TABLE 12 Procedure for Processing DELETE_TS Message from Nbr_ID Extractlist of time slots, Ls, from the DELETE_TS message from Nbr_ID Reset theSlot Assignment DB (mark it as Free) for all time slots in list Ls ResetLink Message state in Link Scheduling Message DB for all time slots inlist Ls for index Nbr_ID Decrement Num_links Return to Idle state

In summary, the objective for the function allocating the semi-permanenttime slots is to connect to as many neighbor nodes as possible up to N.If N neighbor nodes are obtained, then each is allocated a singlesemi-permanent time slot. Once a new link is established by thisprotocol, both nodes will commence operation in the newly allocated SPtime slot.

This operation will test the new link to determine if reliablecommunication can be maintained using the allocated time-slot. Thisinsures that there is no unusual interference that occurs in thisparticular time slot. If the link is tested as unreliable, then thetopology control function will be notified so that the time slot can bedeallocated and used for other purposes.

Allocation of available (on-demand) time slots will now be discussed.The available time slots are to be allocated in a manner that isresponsive to the fluctuating demands of network traffic. Again, assumethat N is fixed and intelligently chosen with, respect to the networksize and environment. Also assume that N_(frame)=2N−1.

To allow fine granularity in the allocation of available capacity, timeslots will be divided into m_(s) sub-time slots. Assume for the rest ofthe following discussion that m_(s)=2. This will be accomplished bydefining a sub-time slot to be a specific time slot allocation thatrepeats every m_(s) ^(th) (or second) frame.

A request for available time slots from one node to a neighbor node isallowed only if at least one semi-permanent time slot is allocated forthe link between these two nodes. After a link is allocated at least onesemi-permanent time slot, then a node may request a periodic allocationof a single time slot every m_(s) ^(th) (or second) frame. The messagesused for scheduling the available time slots can be sent over the PAlink for scheduling time slots several frames in advance of when theyare needed since the link has an allocation of at least onesemi-permanent time slot per frame.

A key requirement for efficient allocation of available time slots isthe measurement of the traffic requirements on each link. Two measureswill be needed. First, the measured average traffic sent over link (i,k) (in units of the number of time slots per frame) will be denoted byT_(ikse). This measure will include all traffic sent over one or moresemi-permanent time slots per frame as well as any available time slots.

In addition, we also need to maintain a current measure of the queuestate, Q_(ik), for link (i, k). Larger values of Q_(ik) indicate theneed for an immediate allocation of one or more available time slots.Occasional bursts of demand may produce increases in Q_(ik), whichshould then trigger a request for additional time slots of on-demandcapacity until the queue size decreases.

The total number of time slots (quantized to ½ of a time slot withm_(s)=2) allocated on link (i, k) will be denoted by N_(ik) ^(tot). Thetime slot demand is defined as follows:T _(ik) ^(dem) =f(T _(ik) ^(se) ,Q _(ik)),  (4)which is a function of the measured traffic plus the estimatedadditional capacity needed that is indicated by the queue size. Then thenumber of time slots needed on this link, T_(ik) ^(need), is as follows:T _(ik) ^(need)=max(T _(ik) ^(dem) , T _(ik) ^(dem))  (5)The metric assigned to this link is as follows: M _(ik) ^(DA) =T _(ik) ^(need) −N _(ik) ^(tot) +B,  (6)which is a measure of the estimated number of additional time slots thatshould be allocated to this link through the DA slot allocationmechanism. B is a bias term that might be nominally set at about ¼ to ½of a time slot to allocated enough excess capacity to each link to avoidsignificant queuing. While we are illustrating the approach using themetric defined in (4), a variety of other forms of metric could also beused as the basis for allocating the DA time slots.

FIG. 8 shows a state diagram of the DA slot assignment process 36. Thestate diagram and the protocol exchanges are similar to those of the SPslot assignment process. In order to simplify the protocol messageprocessing, only a single thread of DA time slot allocation can be inprocess at any time. The idle process does event management in that itchecks received events prior to allowing a state change to one of theother states.

These operations include the following. Check received messages todetermine if they are consistent with the current state of the DB. If amessage is inconsistent with the DB, it is discarded. Certain timeoutsmay indicate that DB state needs to be reset. This process performs thisfunction. It also determines if the DA slot assignment is optimal giventhe traffic load needs of the node. It may cause a transition to the addDA slot state if it determines if a new DA time slot must be added to aparticular link.

There are four basic message types required in the DA time slotassignment protocol as listed below in TABLE 13. These are very similarto those used in the SP slot allocation. The use of these isself-explanatory and consistent with the prior discussion of the SP slotallocation process.

TABLE 13 Message Type Message Function REQ_DATS Request New DA SlotAssignment REPLY_DATS Reply to Received REQ_DATS CONFIRM Response toReceived REPLY_DATS DELETE_TS Message Indicating Deleted Time SlotAllocation LINK_METRIC Message Broadcast to Neighbor Nodes with LinkMetric for Each Link to a Neighbor Node

An example of DA time slot assignment is shown in FIG. 9. Node 1 wantsto add an additional DA time slot allocation for its link (1,2). Thecorresponding protocol message exchange is shown in TABLE 5. Node 1initiates the exchange by sending a REQ_DATS (L=(4.2, 5,6)) indicatingthat it can support allocations of all of slots 5 and 6 and sub-slot4.2. This list may include all free and DA time slots, the later ofwhich are less needed.

When the request message is sent, the appropriate changes are made tothe time slot and link scheduling message data structures. Node 2 isusing time slots 1, 3 and 6 as SP allocations for its links to its 3neighbors and sub-slots 2.1 and 3.2 as DA allocations. It can selecteither sub-slot 4.2 or both sub-slots of slot 5. It chooses and sendsthis choice in the reply message.

When a reply message is sent the appropriate changes are also made tothe time slot and link scheduling message data structures. Finally, whena confirm is sent or received, the state of the appropriate time slotsare changed to “sub-slot 4.2 DA allocated to link (1,2).”

TABLE 14 Node 1 Node 2 Determines That The Link From Node 1 to Node 2Requires An Additional DA Time Slot Send Msg Lost REQ_DATS(L = (4.2, 5,6)) → Timeout and retry Resend → Rcvd REQ_DATS(L = (4.2, 5, 6))REQ_DATS(L = (4.2, 5, 6)) Rcvd REPLY_DATS(Slot 4.2) ← SendREPLY_DATS(Slot 4.2) Send CONFIRM(Slot 4.2) → Rcvd CONFIRM(Slot 4.2)Slot 4.2 DA Allocated Slot 4.2 DA Allocated to Link (1, 2) to Link (1,2)

The following approach is used at each network node to allocate the(N−1) available time slots for directional links to neighbor nodes.Using these measures each node will continuously maintain the linkmetric, M_(ik) ^(DA), for each of its links allocated a semi-permanenttime slot. Each node will use this link metric to indicate the need foradditional transmission time slots to each neighbor node. The largestvalues of M_(ik) ^(DA) indicate the links with the greatest need foradditional on-demand time slot allocation. A positive value of M_(ik)^(DA) indicates the number of additional time slots required, and anegative value of indicates the number of time slots that can besurrendered for reallocation.

As the metrics, M_(ik) ^(DA), are maintained, if the largest link metricindicates a need for an additional sub-slot allocation and if there aresub-slots available either as free slots or as excess DA allocation toother links (again indicated by a small metric), then the processtransitions to the add DA slot state and the process of finding a DAsub-slot allocation is initiated.

As with the semi-permanent time slots, the node need only coordinate theselection of the DA time slot to be assigned for a directional link to aneighbor with that neighbor. This means that a neighbor will send arequest to the neighbor for the time slot assignment over thedirectional link, and receive either a grant of the assignment or adenial of the request over the same link.

Some initial pseudocode describing the processes required in FIG. 8 hasbeen developed. There are various events that may occur which must beprocessed by the DA slot assignment process. Event management is done inthe idle process as shown in TABLE 6.

Four categories of events are shown: 1) received message, 2) checktimeouts, 3) recalculation of link metrics, and 4) DA time slot needsand DA time slot deletion. Received messages are first checked versusthe link scheduling message DB to insure that the message is consistentwith the current state of the DB. For example, if we sent a request to aneighbor, the next message expected is a reply.

To simplify this distributed protocol, only one thread of DA protocolmessage exchanges is allowed at a time. This is enforced in theprocedure by checking the DB to see if other DA message exchanges areongoing prior to initiating an add DA slot transition or prior toprocessing a REQ_DATS message. If an addition slot cannot be initiatedbecause another DA protocol thread is currently in process, the additionslot will not be done.

It can be naturally rescheduled on the next opportunity forrecalculation of link metrics and DA time slot needs. Link metrics willbe recalculated periodically according to a preset-schedule. A linkwhich has a link metric greater than a certain threshold,Max_metric_threshold, is a candidate for obtaining a new DA sub-lot.

The link with the maximum metric that exceeds this threshold will beselected as the next link to which a new DA sub-slot is allocated. Whena new DA sub-slot needs to be allocated and if it satisfies the aboveconditions, then a transition to the add DA slot state occurs in the DAslot assignment process.

TABLE 15 Procedure for Idle State (DA Event Management) Case Event Type Received Message:   If received message is not consistent with thestate of the Link Scheduling Message    DB for that Nbr_ID    DiscardMessage   Elseif message type = REQ_DATS    If no pending DA messageactivity in the Link     Scheduling Message DB for link additions    other than receiving a previous REQ_DATS     message from Nbr_IDTransition to Process     REQ_DATS state to process message    Else    Reject new link and send negative     REPLY_DATS message to Nbr_ID   End   Elseif message type = REPLY_DATS    Transition to ProcessREPLY_DATS state to    process message   Elseif message type = CONFIRM   Transition to Process CONFIRM state to    process message   Elseifmessage type = DELETE_TS    Transition to Process DELETE_TS state to   process message   End  Check Timeouts:   Check all timeouts   IfTimeout expired for a link in the DA_Req state    Transition to Add DASlot state   If Timeout expired for a link in the DA_Reply   state   Reset Slot Assignment DB for time slot Ns and    in the Link Messagestate in     Link Scheduling Message DB for index Nbr_ID   End Recalculate Link Metrics and DA Time Slot Needs:   Recalculate linkmetrics   Send new link metrics to all neighbor nodes in a   LINK_METRICmessage   Sort link metrics and select Largest_link_metric   If (nopending DA message activity in the Link    Scheduling Message DB) and   (Largest_link_metric > Max_metric_threshold)    Transition to Add DASlot state to add new DA    slot assignment to Nbr_ID   End  DA TimeSlot Delete:   Transition to DA TS Delete state to delete Time   Slot toNbr_ID End

Psuedocode for the additional DA slot process is shown in TABLE 16. Thisstarts a process which requires coordination of the time slot assignmentand protocol message exchanges between only the two neighbor nodes. Thenode requesting the link sends a REQ_DATS message to the candidateneighbor node with the list of acceptable time slots for the link.

The list of candidate time slots must contain all free sub-slots and allDA sub-slots with a metric below a certain threshold,Min_metric_threshold. The DA time slots may be currently temporarilyallocated for other DA traffic. This list will be priority-ordered toindicate the sub-slot preference that causes the least perturbation inthe current on-demand time slot assignments. The priority ordering willbe first the free time slots followed by the sub-slots with the smallestmetrics progressing up to the largest metric less than theMin_metric_threshold.

In order to simplify this distributed protocol, only one thread of DAprotocol message exchanges is allowed at a time. This is enforced in theidle procedure. The REQ_DATS message is only sent once, but it could beunsuccessful if the neighbor node is currently processing another DAprotocol exchange. In this case, the node will eventually receive anegative REPLY_DATS message. The attempt to add the DA slot may be madeagain in this case if this link has the largest metric the next time thelink metrics are evaluated. Once the REQ_DATS message is sent theprocess returns to the idle state where other events can be processed.

TABLE 16 Procedure for Addition of a New DA Subslot to the Link to NodeNbr_ID (Generate REQ_DATS Message)  Construct list Ls of time slots(subslots) to offer to Nbr_ID from Free time slots and   DA subslotswith excess capacity (Link_metric < Min_metric_threshold)  Append listLs to REQ_SPTS message and send to Nbr_ID  Setup timeout and LinkMessage state in Link Scheduling Message DB for index   Nbr_ID and inSlot Assignment DB

The neighbor receiving a REQ_DATS message will have its DA slotassignment process transition to the REQ_SPTS state. The procedure forprocessing this message is shown in TABLE 17. This procedure takes theoffered list of sub-slots, Ls, and selects its preferred sub-slot, Ns.The sub-slot accepted is the first sub-slot on the list, ls, that iseither marked free in the slot assignment DB or is DA allocated with alink metric less than Min_metric_threshold. Then a REPLY_DATS replymessage with this selection is sent. If the link cannot be accepted orif there is another ongoing DA slot assignment in process, a negativeREPLY_DATS reply message is sent. The procedure also makes theappropriate modifications to the state in the link scheduling message DBand the slot assignment DB.

TABLE 17 Procedure for Processing REQ_DATS Message (from Nbr_ID) Examine prioritized list Ls of the available subslots  received fromNbr_ID   and compare with the current allocations in   the SlotAssignment DB  Select the best assignment = Ns as the subslot on the list that is either marked Free in   the Slot Assignment DB or is DAallocated with   Link_metric <   Min_metric_threshold If no subslotsatisfies conditions for acceptance  Reject new link and send negativeREPLY_DATS message to  Nbr_ID  Return to Idle state Else  Makeappropriate modification to the Slot Assignment DB  (mark it asDA_Reply)   for time slot Ns  If time slot Ns was DA allocated   SendDELETE_TS to the neighbor node allocated the   DA time slot  End  Appendtime slot choice, Ns, to REPLY_DATS message and  send to Nbr_ID  Setuptimeout and Link Message state (to DA_Reply with  time slot Ns) in Link  Scheduling Message DB for index Nbr_ID  Return to Idle state End

A received REPLY_DATS message is processed as shown in TABLE 18. Thechoice of sub-slot, Ns, received from the neighbor node is extractedfrom the message. We require the node to confirm this reply with eithera positive or negative CONFIRM message that indicates that it will agreeto use the allocated time slot. As indicated in the SP allocationprocess, this three-way handshake eliminates uncertainty in the outcomeof the scheduling process.

If the REPLY_DATS message is a positive reply, then the choice ofsub-slot, Ns, is examined to see if it is still an allowable assignmentfor a new DA sub-slot for the new link. If it is allowable, then theappropriate modifications to the state in the slot assignment and linkscheduling message databases are made. Then a positive CONFIRM messageis returned.

If the received REPLY_SPTS message was negative, then the slotassignment and link scheduling message databases are reset for thisNbr_ID. Otherwise, if the choice of Ns is no longer allowable, then thelink scheduling message database is reset for this Nbr_ID. Then anegative CONFIRM message is sent to the neighbor node rejecting thelink.

TABLE 18 Procedure for Processing REPLY_DATS Message from Nbr_ID Extracttime slot choice Ns from the REPLY_DATS message from Nbr_ID If (positiveREPLY_DATS message) and (choice of Ns is still allowable from Slot Assignment DB)  Make appropriate modification to the Slot Assignment DB (mark it as DA_Reply)   for time slot Ns and in the Link Message statein   Link Scheduling Message DB   for index Nbr_ID  If time slot Ns wasDA allocated   Send DELETE_TS to the neighbor node allocated the   DAtime slot  End  Create CONFIRM message for Ns and send to Nbr_ID  Returnto Idle state Elseif negative REPLY_DATS message  Reset Slot AssignmentDB for time slot Ns and in the  Link Message state in   Link SchedulingMessage DB for index Nbr_ID  Return to Idle state Else  Reset LinkMessage state in Link Scheduling Message DB  for index Nbr_ID  Sendnegative CONFIRM message to Nbr_ID  Return to Idle state End

TABLE 19 shows the procedure for processing CONFIRM messages. If theCONFIRM is positive, the selected sub-slot to be added to the allocationto the link to Nbr_ID. The assigned time slot, Ns, is marked DA_Alloc inthe slot assignment DB, and the link message state in the linkscheduling message DB is reset for index Nbr_ID. If the message was anegative CONFIRM, then the slot assignment and link scheduling messagedatabases are reset for this sub-slot.

TABLE 19 Procedure for Processing CONFIRM Message from Nbr_ID Ifpositive CONFIRM message  Make appropriate modification to the SlotAssignment DB  (mark it as DA_Alloc)   for time slot Ns  Reset LinkMessage state in Link Scheduling Message DB  for index Nbr_ID  Return toIdle state Else  Reset the Slot Assignment DB (mark it as Free) for time slot Ns  Reset Link Message state in Link Scheduling Message DB  forindex Nbr_ID  Return to Idle state End

An allocated time slot may need to be deallocated for one of severalreasons. If during the course of normal operation a link goes down orbecomes unreliable, then the topology control function gets involved toaddress the unreliable link problem. Ultimately, it may generate atopology change (e.g., a link deletion) event directing the SP slotassignment process to delete all slots assigned to the link.

The steps involved in this procedure are shown in TABLE 11. The link isde-allocated by sending a DELETE_TS message from the node requesting thede-allocation of all the time slots which are shared with the other nodewith. In addition, the appropriate entries in the link schedulingmessage DB and the slot assignment DB are reset.

TABLE 20 Procedure for DA TS Delete to Node Nbr_ID (Generate DELETE_TSMessage) Construct message, DELETE_TS, containing the DA subslot, Ns,that is to be deleted  and send to Nbr_ID Reset Link Scheduling MessageDB for index Nbr_ID and Slot  Assignment DB for subslot Ns Return toIdle state

Table 21 shows the procedure for processing a received DELETE_TSmessage. The subslot, Ls, to be deallocated is extracted from themessage. Then the appropriate state in the slot assignment DB and in thelink scheduling message DB is reset.

TABLE 21 Procedure for Processing DELETE_TS Message from Nbr_ID ExtractDA subslot, Ns, from the DELETE_TS message from Nbr_ID Reset the SlotAssignment DB (mark it as Free) for subslot Ns Reset Link Message statein Link Scheduling Message DB for subslot Ns Return to Idle state

The link scheduling algorithm is also applicable to multiplesimultaneous beams generated by the phased array antenna 16. Assume theextension to a system with nodes each employing multiple antenna beamswith separate receivers such as a multiple beam phased array (or othertypes of multiple, directional antennas). Furthermore, assume that allnodes do not all have to have the same number of beams, i.e., node k hasB_(k) beams. This is equivalent to B_(k) parallel links possible at anytime slot.

We are extending the previous discussion (which assumed a single steeredbeam) to allow the B_(k) beams to be time-shared among a set of neighbornodes larger than B_(k). Even though the nodes may each have differentnumbers of beams, all nodes must use a common time slot format and framewith a number of time slots per frame for each beam equal to N_(frame).

Consider an upper limit at any node k on the number of semi-permanently(SP) assigned time slots on any one of its B_(k) beams (and thereforethe maximum number of allowable neighbor nodes per beam) to be denotedby N_(beam). The value of N_(beam) is dependent only on the number oftime slots per frame and not the number of beams. As in (3) we willspecify that N_(beam) must satisfy the following equation:N _(frame)≧2·N _(beam) ⁻¹  (7)

Assume that all nodes in a network are connected by directional links,where node k has B_(k) beams with beam sharing by time hopping andpointing to its neighbor nodes. Further, assume the number of neighborsallowed per beam is equal to N_(beam), the fixed limit on the allowablenumber of semi-permanent time slots allowed per beam (with one SP timeslot allocated per neighbor).

If the fixed value of N_(beam) for each beam at each neighbor nodesatisfies (7), then all nodes can select a different semi-permanent timeslot for each of these links and each of its beams by mutual agreementwith the neighbor for that link without regard to what colors othernodes are selecting more than one hop away. This allows each node toselect its N_(beam) semi-permanent time slots for each beam in a verydirect fashion by communicating only with its neighbor node. Byfollowing this strategy, each node is able to support at least

 N _(k) =B _(k) ·N _(beam)  (8)

neighbors and each allocated a single SP time slot with no more thanN_(beam) such time slots allocated per beam.

Verification that N_(beam) neighbors per beam can be supported as longas (7) is satisfied follows directly from the verification of theobservation for the single beam case. Then if all B_(k) beams have theirSP time slots scheduled in the same fashion, it is obvious that thenumber of neighbor nodes that can be supported is the product of thenumber of beams and the number of neighbors per beam resulting in (8).

An example of SP time slot assignment between two nodes with an unequalnumber of beams per node is shown in FIG. 10. In this example node 1 has2 beams and node 2 has 3 beams. While the two nodes have differentnumbers of beams, both nodes must use the same frame structure. In thisexample N_(frame)=5 time slots per frame. From (7) and (8), this allowsnode 1 to have a maximum of 6 neighbors and node 2 to have a maximum of9 neighbors.

Initially both nodes have one less than the maximum number of neighborsthey are allowed under the constraints of (7) and (8). The SP beam/timeslots allocations are shown for each link. These nodes can add anadditional link between themselves while still satisfying theconstraints of (7) and (8). The link scheduling protocol will find anacceptable beam/time slot for the SP allocation for each node, and itoperates in essentially the same way it did with the single beam case.

The corresponding protocol message exchange is shown in TABLE 22. Node 1initiates the exchange by sending a REQ_SPTS(L=(1, 2, 3)) with a list ofat least N_(beam) candidate time slots. Note the 3 beam IDs are denotedby a, b and c, and the slot number is denoted by the subscript on thebeam ID. Node 1 had to identify that it had used all 3 allowable SP timeslots on beam a, but it had allocated only 2 of the 3 allowable SP timeslots on its beam b.

Thus, it sent a list of the 3 SP time slots (available on beam b) tonode 2. This list may include all free and DA time slots on this beam.When the request message is sent, the appropriate changes are made tothe time slot and link scheduling message data structures. Node 2 haspreviously allocated SP all available SP time slots on beams a and b forits links to its 8 neighbors.

Thus, beam c is the only beam that can accept a new SP allocation. Whenit receives the REQ_SPTS(L=(1, 2, 3)) from node 1, it selects beam/timeslot c₃ as the only one that will work for the new link (havingpreviously allocated c₁ and c₂ as SP time slots). It sends this choicein the reply message. When a reply message is sent the appropriatechanges are also made to the beam/time slot and link scheduling messagedata structures. Finally, when a confirm is sent or received, the stateof the appropriate time slots are changed to “SP allocated to link(1,2).”

TABLE 22 Node 1 Node 2 Receives Link Add Event From Its Topology ControlFor A Link From Node 1 to Node 2 Send REQ_SPTS (L = (1, 2, → Rcvd Send3)) REQ_SPTS(L = (1, 2, 3)) Rcvd REPLY_SPTS (Slot 3) ← Send REPLY_SPTS(Slot 3) Send CONFIRM (Slot 3) ∴ Rcvd CONFIRM (Slot 3) Beam/Slot b₃Beam/Slot c₃ Allocated to Link (1, 2) Allocated to Link (1, 2)

The changes that are required to implement the multiple beam schedulingalgorithm/protocol are straightforward and are as follows. Add the beamID as a variable in the state of the time slot DB and the linkscheduling message DB. Use (7) and (8) as the criteria for determiningif it is possible to schedule a new SP time slot. We specify a value forthe parameters N_(frame) and N_(beam) for the network.

To offer a new SP time slot to a potential neighbor, the algorithm mustfirst find a beam for which the number of neighbors is less thanN_(beam). This beam can then be used to add the new neighbor. TheREQ_SPTS message that the node sends to its neighbor will specifyN_(beam) available time slots for that beam that are not currently SPallocated.

Having received an REQ_SPTS message the node must find one of its beamsfor which the number of neighbors is less than N_(beam). This beam canthen be used to add the new neighbor. Comparing the list of N_(beam)time slots in the received REQ_SPTS message with the N_(beam) time slotsnot currently allocated in the selected beam, at least one time slot canbe found that is common to both lists. That time slot can be selected asthe time slot to send in the REPLY_SPTS message. Once the originatingnode receives the REPLY_SPTS message, both nodes will have selectedtheir beam and the common time slot allocation.

This example implicitly assumed that a single frequency band is used foreach of the beams. In this case, a node could have several beamssimultaneously communicating over the same band without interference.This interference-free operation may be difficult to support inpractice. A similar formulation of the problem could be done with eachbeam operating in a different frequency band, i.e., beams a, b, and c inFIG. 10 each use a different frequency band. In terms of the schedulingalgorithm, we would apply the same constraints on the allocation of SPtime slots. However, in actually allocating the time slot/beamcombinations we would need to find an allocation such that the two nodesare using the same beam (equivalent to using the same band) as well asthe same time slot. This equivalent to making each beam/time slotcombination different from the scheduling perspective. Thus, the numberof available time slots is the number of beams multiplied by the framesize. In this case the constraint on assigning SP time slots topotential neighbors is given by

 B·N _(frame)≧2·N−1,  (9)

where B denotes the number of beams. This constraint on the number ofneighbors is slightly more restrictive than that of (7) and (8) becauseof the requirement that nodes which share an SP time slot must also usethe same beam/frequency channel as well as the same time slot. For theexample N_(frame)=5 and B=3, then the constraint of (9) allows 8neighbors for each node whereas the constraints of (7) and (8) willallow 9 neighbors for each node.

The example problem in FIG. 10 has 2 nodes each with 3 beams with eachbeam operating in a different frequency band, i.e., beams a, b, and ceach use a different frequency band. Assume also that the frame size is5. Both nodes have already committed 7 SP time slots to neighbor nodesand thus, from (9), they can each add an additional neighbor with an SPtime slot allowing them to establish a link between them. The committedSP time slots are indicated in the figure, and the message exchangesrequired to establish the SP time slot assignment and the new link areindicated in Table 23. The message exchange is initiated by node 1 bysending a REQ_SPTS (L=(a₄, a₅, b₃, b₄, b₅, c₃, c₄, c₅)) message to node2 which must include the 8 beam/time slot combinations it has notpreviously allocated as SP time slots. In this example, node 2 hadalready allocated 7 beam/time slot combinations that were not used bynode 1 (which were in the list of 8 beam/time slot combinations receivedin the REQ_SPTS message). Thus, by (9) there must be at least oneremaining beam/time slot combination that it can select for allocation(c₅). This is the SP beam/time slot combination allocated to the linkbetween nodes 1 and 2 as show in both FIG. 11 and Table 23.

TABLE 23 Node 1 Node 2 Receives Link Add Event From Its Topology ControlFor A Link From Node 1 to Node 2 Send REQ_SPTS (L = (a₄, → Rcvd Send a₅,b₃, b₄, b₅, c₃, c₄, REQ_SPTS (L = (a₄, a₅, c₅)) b₃, b₄, b₅, c₃, c₄, c₅))Rcvd REPLY_SPTS ← Send REPLY_SPTS (Beam/Slot c₅) (Beam/Slot c₅) SendCONFIRM (Beam/Slot → Rcvd CONFIRM c₅) (Beam/Slot c₅) Beam/Slot c₅Beam/Slot c₅ Allocated to Link (1, 2) Allocated to Link (1, 2)

The present invention thus provides a fully distributed link schedulingalgorithm and protocol for phased array networks. The description of thealgorithm/protocol details assumed the case of a single directional beamper node, which is time-shared and pointed toward neighbor nodes duringthe allocated time slot for that access. However, the approach can beused for an arbitrary number of steered beams per node.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed, and that modificationsand embodiments are intended to be included within the scope of theappended claims.

1. A wireless communication network comprising: a plurality of mobilenodes each comprising a transceiver, a directional antenna connected tosaid transceiver, and a controller connected to said transceiver andcomprising a first time slot unit to schedule a respectivesemi-permanent time slot for each time frame to establish acommunication link with each neighboring mobile node and leaving atleast one available time slot in each time frame, a second time slotunit to schedule the at least one available time slot to also serve thecommunication link with a neighboring mobile node based upon linkcommunications demand, an antenna aiming unit to aim said directionalantenna toward each neighboring mobile node during communicationtherewith, and an interference detection unit to detect interference intime slots for communication with neighboring mobile nodes.
 2. Awireless communication network according to claim 1, wherein saidinterference detection unit measures a signal-to-interference ratio. 3.A wireless communication network according to claim 1, wherein saidinterference detection unit measures a packet error rate.
 4. A wirelesscommunication network according to claim 3, wherein the packet errorrate is based upon cyclic redundancy check (CRC) failures.
 5. A wirelesscommunication network according to claim 1, wherein said interferencedetection unit compares detected interference to a threshold.
 6. Awireless communication network according to claim 1, wherein saidcontroller switches an order of communication between nodes for a giventime slot based upon detected interference.
 7. A wireless communicationnetwork according to claim 6, wherein said controller furthercoordinates scheduling of new time slots based upon detectedinterference.
 8. A wireless communication network according to claim 1,wherein said controller coordinates scheduling of new time slots basedupon detected interference.
 9. A wireless communication networkaccording to claim 1, wherein a plurality of communication links areestablished within a scheduled semi-permanent time slot, with eachcommunication link including a different pair of neighboring mobilenodes.
 10. A wireless communication network according to claim 1,wherein said directional antenna comprises a phased array antenna.
 11. Awireless communication network comprising: a plurality of mobile nodeseach comprising a transceiver, a directional antenna connected to saidtransceiver, and a controller connected to said transceiver andcomprising a time slot scheduling unit to schedule time slots toestablish a communication link with each neighboring mobile node, anantenna aiming unit to aim said directional antenna toward eachneighboring mobile node during communication therewith, and aninterference detection unit to detect interference in time slots forcommunication with neighboring mobile nodes, said controllercoordinating the scheduling of time slots based upon detectedinterference.
 12. A wireless communication network according to claim11, wherein said interference detection unit measures asignal-to-interference ratio.
 13. A wireless communication networkaccording to claim 11, wherein said interference detection unit measuresa packet error rate.
 14. A wireless communication network according toclaim 13, wherein the packet error rate is based upon cyclic redundancycheck (CRC) failures.
 15. A wireless communication network according toclaim 11, wherein said interference detection unit compares detectedinterference to a threshold.
 16. A wireless communication networkaccording to claim 11, wherein said controller switches an order ofcommunication between nodes for a given time slot based upon detectedinterference.
 17. A wireless communication network according to claim16, wherein said controller coordinates scheduling of new time slotsbased upon detected interference after switching the order ofcommunication.
 18. A wireless communication network according to claim11, wherein a plurality of communication links are established within ascheduled semi-permanent time slot, with each communication linkincluding a different pair of neighboring mobile nodes.
 19. A wirelesscommunication network according to claim 11, wherein said directionalantenna comprises a phased array antenna.
 20. A mobile node for awireless communication network, the mobile node comprising: atransceiver; a directional antenna connected to said transceiver; and acontroller connected to said transceiver and comprising a time slotscheduling unit to schedule time slots to establish a communication linkwith neighboring mobile nodes, an antenna aiming unit to aim saiddirectional antenna toward each neighboring mobile node duringcommunication therewith, and an interference detection unit to detectinterference in time slots for communication with neighboring mobilenodes, said controller coordinating the scheduling of time slots basedupon detected interference.
 21. A mobile node according to claim 20,wherein said interference detection unit measures asignal-to-interference ratio.
 22. A mobile node according to claim 20,wherein said interference detection unit measures a packet error rate.23. A mobile node according to claim 22, wherein the packet error rateis based upon cyclic redundancy check (CRC) failures.
 24. A mobile nodeaccording to claim 20, wherein said interference detection unit comparesdetected interference to a threshold.
 25. A mobile node according toclaim 20, wherein said controller switches an order of communicationbetween nodes for a given time slot based upon detected interference.26. A mobile node according to claim 25, wherein said controllercoordinates scheduling of new time slots based upon detectedinterference after switching the order of communication.
 27. A mobilenode according to claim 20, wherein a plurality of communication linksare established within a scheduled semi-permanent time slot, with eachcommunication link including a different pair of neighboring mobilenodes.
 28. A mobile node according to claim 20, wherein said directionalantenna comprises a phased array antenna.
 29. A method for establishingcommunication links for a plurality of mobile nodes, each mobile nodecomprising a transceiver, a directional antenna connected to thetransceiver, and a controller connected to the transceiver, the methodcomprising for each mobile node: scheduling a respective semi-permanenttime slot for each time frame to establish a communication link with aneighboring mobile node and leaving at least one available time slot ineach time frame; scheduling the at least one available time slot to alsoserve the communication link with a neighboring mobile node based uponlink communications demand; aiming the directional antenna toward eachneighboring mobile node during communication therewith; detectinginterference in time slots for communication with neighboring mobilenodes; and coordinating the scheduling of new time slots based upondetected interference.
 30. A method according to claim 29, whereindetecting interference comprises measuring a signal-to-interferenceratio.
 31. A method according to claim 29, wherein detectinginterference comprises measuring a packet error rate.
 32. A methodaccording to claim 31, wherein the packet error rate is based uponcyclic redundancy check (CRC) failures.
 33. A method according to claim29, wherein detecting interference comprises comparing detectedinterference to a threshold.
 34. A method according to claim 29, furthercomprising switching an order of communication between nodes for a giventime slot based upon detected interference.
 35. A method according toclaim 29, wherein a plurality of communication links are establishedwithin a scheduled semi-permanent time slot, with each communicationlink including a different pair of neighboring mobile nodes.